Probably the Beast, but skip the pointless fire road loop of pointlessness about halfway around. It's just after the house/cafe that's partway round and a quick look at a map will show you how a few minutes of steep road and fire road climbing can avoid a long, dull fire road climb and descent which doesn't even have particularly good views.

ChrisL and timmys are on the money - the Beast is definitely the best way to spend a day there, with lunch at the halfway café if possible. Afterwards, carry on straight up the [steep] tarmac road then take the second fire road on the right to cut out a big unnecessary FR loop.

[Pedant mode] Coed just means "wood" in Welsh so as you'd expect there are lots of riding spots with coed in their name - I hope we were all right in assuming you meant Coed y Brenin though?